<!DOCTYPE HTML>
<html>
  <body>

    <!-- Stretchy fences -->

    <math>
      <mfenced> 
        <mtable>
          <mtr>
            <mtd><mi>a</mi></mtd>
            <mtd><mi>b</mi></mtd>
          </mtr>
          <mtr>
            <mtd><mi>c</mi></mtd>
            <mtd><mi>d</mi></mtd>
          </mtr>
        </mtable>
      </mfenced>
    </math>

  </body>
</html>
